Subterranean heaters have been used to heat subterranean geological formations in oil production, remediation of contaminated soils, accelerating digestion of landfills, thawing of permafrost, gasification of coal, as well as other uses. Some examples of subterranean heater arrangements include placing and operating electrical resistance heaters, microwave electrodes, gas-fired heaters or catalytic heaters in a bore hole of the formation to be heated. Other examples of subterranean heater arrangements include circulating hot gases or liquids through the formation to be heated, whereby the hot gases or liquids have been heated by a burner located on the surface of the earth. While these examples may be effective for heating the subterranean geological formation, they may be energy intensive to operate.
U.S. Pat. Nos. 6,684,948 and 7,182,132 propose subterranean heaters which use fuel cells as a more energy efficient source of heat. The fuel cells are disposed in a heater housing which is positioned within the bore hole of the formation to be heated. The fuel cells convert chemical energy from a fuel into heat and electricity through a chemical reaction with an oxidizing agent. U.S. Pat. Nos. 6,684,948 and 7,182,132 illustrate strings of fuel cells that may be several hundred feet in length. Operation of the fuel cells requires fuel and air to be supplied to each of the fuel cells and spent fuel (anode exhaust) and spent air (cathode exhaust) must be exhausted from each of the fuel cells. In order to do this, a fuel supply conduit and an air supply conduit are provided such that each extends the entire length of the string of fuel cells to supply fuel and air to each of the fuel cells. Similarly, an anode exhaust conduit and a cathode exhaust conduit are provided such that each extends the entire length of the string of fuel cells to expel anode exhaust and cathode exhaust from each of the fuel cells to the surface (e.g., the top of the bore hole of the formation).
U.S. patent application Ser. No. 14/013,708 to Fischer et al., the disclosure of which is incorporated herein by reference in its entirety, teaches a subterranean heater which uses fuel cells and combustors to heat a geological formation. The fuel cells and combustors are disposed in a heater housing in an alternating pattern and are operated to heat the heater housing, and consequently the geological formation. However, the fuel cells cannot utilize all of the hydrogen present in the reformate that is supplied as a fuel to the anode of the fuel cell because a typical safe fuel utilization range is between about 40% and about 60%. Higher fuel utilization may lead to decreased durability of the fuel cells, consequently, there is a significant amount of hydrogen remaining in the anode exhaust. Similarly, there is oxygen remaining in the cathode exhaust. In order to utilize the remaining hydrogen from the anode exhaust and the remaining oxygen from the cathode exhaust, the combustors are supplied with anode exhaust and cathode exhaust from the fuel cells. The anode exhaust and the cathode exhaust are mixed within the combustor and the mixture is combusted to produce a heated combustor exhaust which is discharged into the heater housing. While this arrangement may utilize more of the hydrogen in order to produce heat for heating the geological formation, the combustor fuel flow is dependent on the reformate flow to the fuel cells and consequently cannot be controlled independently of the fuel cells; however, it may be desirable to adjust the thermal output of the combustors independently of the fuel cells.
What is needed is a heater which minimizes or eliminates one of more of the shortcomings as set forth above.